Manifold forms of such boarding and/or entry aids are known. They are used in vehicles that carry passengers, such as public transit buses and rail cars, and they make it easier to board such vehicles and disembark from them by providing that a displaceable and optionally also pivotable step plate on each vehicle, in the form of a ramp, linear step, or the like, is deployed outward in the vicinity of transit stops. The term boarding and/or entry aid is understood also to include its use as an aid in disembarking from and exiting a vehicle.
The step plate, for instance of aluminum and provided with a slip-reducing covering, is coupled by its back side to a rolling carriage that is supported displaceably in rails mounted below the deck of the vehicle. They are preferably positioned, before a vehicle door is opened, between a standing and waiting area for passengers to be carried, such as a platform, and a deck surface in the entry area of the vehicle, so that any differences in height between these two areas and/or gaps between them can be compensated for or made more easily passable. This makes it possible for instance for wheelchair users and people pushing strollers to board and disembark easily. At the end of the disembarkation and boarding phase and optionally after the vehicle doors have been closed, the step plate is retracted to its outset position.
So that the step plate, together with the associated functional parts, can be installed as a complete structural unit, the rails for the rolling carriage are secured to the lateral parts of a rectangular frame, on which the drive means required for moving the rolling carriage back and forth are also installed. The frame overall comprises a welded construction and is mounted underneath the deck of the vehicle.
For exact tracking, the lateral parts and the rails secured to them must be oriented exactly parallel to one another. This is especially true if, to reduce production costs, it is desired that the rolling carriage be provided with only a single drive mechanism, functioning in the middle between the lateral parts.
Because of the requisite welding operations in assembling the lateral parts, end profiles and bracing struts that form the frame, exact parallelism cannot be ensured. For the same reasons, it is difficult to adhere to specified tolerances with regard to the length and width of the frame. It is therefore necessary that the parallelism of the rails and/or smooth displaceability of the rolling carriage be ensured in some other way. To that end, on the one hand, additional work steps are needed in the production and/or assembly of the frame, which is associated with an unwanted increase in production costs. On the other hand, comparatively great production tolerances must be accepted into the bargain, which is likewise as a rule unwanted.